The present invention relates to a polypropylene random copolymer and a film thereof. More particularly, the invention relates to a polypropylene random copolymer and a film thereof excellent in an appearance and excellent in balance of low-temperature heat-sealing property and rigidity.
Since a polypropylene film is excellent in appearance, heat-seal characteristics, heat resistance, rigidity, etc., the film has been widely used in a packaging field such as food packaging, fiber packaging, etc.
Recently, for improving productivity in the packaging field, production speed at a bag-making or packaging has been increased and for this purpose, development of materials excellent in higher speed bag-making property and packaging property and showing a good sealing property at low temperature has been desired. Thus, various investigations have been made to improve the heat sealing property at low temperature, and a method of compounding various kinds of low-crystalline ethylene-propylene copolymers with a polypropylene random copolymer and a method of increasing content of ethylene and/or an xcex1-olefin in a polypropylene random copolymer have been attempted. However, those products obtained are yet insufficient in the point of a polypropylene random copolymer and the film thereof excellent in appearance and also excellent in a balance of low-temperature heat sealing property and rigidity. That is, if the amount of the low-crystalline polypropylene random copolymer compounded or the content of the comonomer such as ethylene, etc., in a random copolymer is increased to attain the low-temperature heat-sealing property, the rigidity thereof is greatly decreased or a phenomenon such as a poor appearance by whitening due to bleeding occurs, whereby the desired object cannot be attained.
A solution polymerization method or a bulk polymerization method has been known as the production method of a polypropylene random copolymer.
In those polymerization methods, since the copolymerization or the post treatment of the copolymer is conducted in a solvent such as heptane, hexane, etc., or a mixed medium of propylene, an xcex1-olefin and/or ethylene, low-molecular weight polymer components giving influences on the blocking resistance, whitening property by bleeding, etc., are dissolved and removed to give preferred characteristics.
However, on the other hand, problems occur that the colopolymer components effective for the heat sealing property at low temperature are dissolved and removed and if the content of the comonomer such as an xcex1-olefin and/or ethylene is intended to increase, the copolymer itself is dissolved and thus cannot be produced. For those reasons, if it is intended to obtain a material capable of heat sealing at lower temperature by those polymerization methods, there is a large restriction and the production of such a copolymer by an industrially useful method has not substantially been attained.
Thus, for the purpose of attaining the above object, it has been attempted to produce the copolymer by a so-called gas-phase polymerization method in which a solvent or an active monomer liquid is not substantially present.
Methods described in JP-A-60-166455, JP-A-53-26882, etc., (the term xe2x80x9cJP-Axe2x80x9d as described herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) are known as this method, but in the point to obtain the polypropylene random copolymer and the film thereof excellent in the appearance and also excellent in the balance of the low-temperature heat sealing property and the rigidity, the products obtained are yet insufficient.
Accordingly, an object of the present invention is to provide a polypropylene random copolymer and a film thereof excellent in appearance and also excellent in the balance of the low-temperature heat sealing property and the rigidity without deterioration of the transparency which is the preferred characteristic the film of polypropylene inherently possesses.
As a result of various investigations to develop a polypropylene random copolymer and film thereof excellent in appearance and also excellent in the balance of the low-temperature heat sealing property and the rigidity without deterioration of the transparency which is the characteristic the film of polypropylene inherently possesses, which have not been obtained by the above-described conventional techniques, the inventors have found that the above object can be attained by a polypropylene random copolymer wherein the copolymer compositions are in specific ranges and the contents of 20xc2x0 C. xylene-soluble components are in specific ranges. The present invention has been accomplished based on this finding.
That is, according to one embodiment of the present invention, there is provided a polypropylene random copolymer obtained by copolymerizing propylene and an xcex1-olefin, or propylene, ethylene, and an xcex1-olefin substantially in the absence of a solvent using a Ziegler-Natta catalyst, wherein the propylene content is from 92.3 to 75.0% by weight and, ethylene content (E) is from 0 to 2.7% by weight, and the xcex1-olefin content is from 5.0 to 25.0% by weight, and wherein the content (C) of 20xc2x0 C. xylene-soluble components in the random copolymer satisfies the following formulae (1) to (3);
in the copolymer wherein the ethylene content (E) is less than 1.5% by weight,
Cxe2x89xa612.0 (weight %)xe2x80x83xe2x80x83(1)
in the copolymer wherein the ethylene content (E) is from 1.5% by weight to 1.8% by weight,
Cxe2x89xa642.0xe2x88x9220.0xc3x97E (weight %)xe2x80x83xe2x80x83(2)
and in the copolymer wherein the ethylene content (E) is from 1.8% by weight to 2.7% by weight,
xe2x80x83Cxe2x89xa66.0 (weight %)xe2x80x83xe2x80x83(3).
According to another embodiment of the present invention, there is provided a polypropylene film formed by melt-extruding the above-described polypropylene random copolymer.